Unusual internal rotation coupling in the microwave spectrum of pinacolone

Abstract

The molecular beam Fourier-transform microwave spectrum of pinacolone (methyl tert-butyl ketone) has been measured in several regions between 2 and 40 GHz. Fits of the assigned spectrum using several computer programs based on different models for treating torsion-rotation interaction lead to the conclusion that no existing program correctly captures the internal dynamics of this molecule. Quantum chemical calculations at the MP2/6-311++G(d,p) level of theory indicate that this molecule does not have a plane of symmetry at equilibrium, and that internal rotation of the light methyl group induces a large oscillatory motion of the heavy tert-butyl group from one side of the Cs configuration to the other. This effect has been modeledfor J = 0 levels by a relatively simple two-top torsional Hamiltonian, where the magnitudes of the strong coupling terms between the tops are determined directly from the ab initio two-dimensional potential surface. A plot of the resultant 0A, 0E, 1E, 1A torsional levels on the same scale as a one-dimensional potential curve along the zig-zag path connecting the six (unequally spaced) minima bears a striking resemblance to the 1:2:1 splitting pattern of the A, E, E, B levels of an internal rotation problem with a six-fold barrier. It seems likely that rotational transitions within the 1E and 1A torsional levels are the cause of the roughly 50% of the spectrum that remains unassigned after all predicted transitions within the 0A and 0E torsional levels are removed. However, a much more complete measurement campaign and some new torsion-rotation theory will be needed to verify this hypothesis.

Sensing the Molecular Structures of Hexan‐2‐one by Internal Rotation and Microwave Spectroscopy

“…Conformer I with a straight butyl chain seems to possess C s symmetry, but the dihedral angles optimized at the MP2 and B3LYP levels shown in Table and reveal that the whole butyl group is slightly tilted out of the C−(C=O)−C plane by an angle of about 10° ( θ 1 =169.4°) and 5° ( θ 1 =174.5°), respectively. The same phenomenon could be observed in many other aliphatic ketones, as for example in diethyl ketone, pinacolone, and methyl neopentyl ketone with respective tilt angles of approximately 12°, 16°, and 10°. Moreover, this tilt‐distorted C s structure, hereafter called “pseudo‐C s ”, is also found for the “straight“ conformers of the shorter linear methyl alkyl ketones, methyl ethyl ketone (9°) and methyl propyl ketone (10°).…”

2-Propionylthiophene: planar, or not planar, that is the question

“…While quantum chemistry sometimes predicts that the alkyl group(s) should be tilted out of the C-(C=O)-C plane by a small angle, typically from 5° to 15°, a model for an effective planar molecule could be applied with success to reproduce the experimental spectra. Similar problems were encountered in investigations on aliphatic ketones with longer carbon chains such as pentan-2-one, 7 hexan-2-one 8 and heptan-2-one, 9 or even when the alkyl chain is branched as in the cases of pinacolone 10 and methyl neopentyl ketone. 11 A conclusive statement could not be drawn since the experimental data did not allow to confirm or deny a Cs structure, as no isotopic species could be measured.…”

The effect of internal rotation in p-methyl anisole studied by microwave spectroscopy

“…The V6 contribution in the potential would generate additional splittings of the A and E torsional species of the three-fold rotor if the lowest torsional level (18.85 cm -1 in the case of PMA, value calculated by the program BELGI-Cs) lies below the local maxima, as observed for pinacolone [25]. Because no additional splittings were observed experimentally, it is most likely not the case.…”